Low rank coals, such as lignite and sub-bituminous coal are readily available. They may, however, have such high moisture contents and low heating values that they cannot be used as fuels in existing boilers without derating or significant modifications. These coals can be upgraded by thermal drying to reduce the moisture contents and heating values sufficiently that the dried coals may compete favorably with many bituminous coals. With a low sulfur content such coals can meet clean air requirements for many power plants without new flue gas desulfurization systems and make a major contribution to reducing sulfur dioxide emissions and acid rain. The drying required with such low rank coals is a deep drying process which removes both surface water and large quantities of interstitial water present. The handling, storage and transportation of such deep dried coals can present technical problems resulting from the friability and dustiness of the coals, as well as their tendencies to readsorb moisture and react with oxygen from the air. Spontaneous combustion can result from heats of moisture readsorption and oxidation. Removing moisture inherent in the coals structures can also reduce the strength of the coal particle by cracking or fissuring, causing friability and dusting. The number of active surface sites exposed within the coal particles can also thus be increased, thereby increasing undesired moisture adsorption and oxidation.
Drying conditions such as temperature, residence time within the drying chamber, drying gas velocities, etc., affect the tendencies of the dried coal product to exhibit the undesirable qualities discussed above. For example, rapid removal of moisture by a high drying temperature can cause what is commonly called "the popcorn effect"--the fissuring and cracking and disintegration of the coal particles. Drying the coal and removing the moisture at a slower rate can reduce this effect--preventing disintegration and allowing moisture to escape in a manner that reduces cracking and fissuring. Smaller fissures make the coal particles more amenable to surface treatment agents which block the pores or coat the particle surfaces and act as effective oxidation or moisture readsorption inhibitors. However, slower drying rates necessitate longer residence times in the drying chamber to achieve the same degree of moisture removal. Because of the turbulent action in a fluidized drying bed, longer residence time leads to increased mechanical size-degradation of the coal particles, increasing dust in the dried product.